1. Field of the invention
The invention relates to a method and a system for controlling engine speed and more particularly to a method for controlling engine speed when a vehicle having such engine transitions from a rest condition to a moving condition (i.e., a vehicle launch).
2. Background of the invention
As is known in the art, with a vehicle equipped with a manual transmission, during a typical, driver-actuated gearshift event, the driver matches the torque during the phases of the shift by adjusting the accelerator pedal position. The pedal position actuates either mechanically or electronically, the intake throttle valve.
As is also known in the art, with an Automatic Shift Manual (ASM) transmission the traditional manual gearshift lever is replaced with operator hand-operated up-shift and down-shift paddles which are part of a driver interface. The ASM transmission uses a sophisticated electro-mechanical control system to eliminate clutch pedal control by the operator of the vehicle completely. More particularly, with an ASM transmission, the driver makes gear selections with the easy-to-operate electronic paddles while the vehicle""s control system executes the driver""s decision. During such execution, the system co-ordinates all gear-change events including engine torque ramp-down and ramp-up. An ASM transmission is an automatic manual gearbox because the mechanical linkages, which would commonly be controlled by the operator of the vehicle in a manual transmission, are supplanted by high-speed electrohydraulic actuators.
There are two operator selectable modes by which an ASM transmission can operate: (1) An Operator Select-ASM (OS-ASM) mode; and (2) an Automatic Select-ASM (AS-ASM). The particular mode is selected by the operator by pressing one of a pair of operator hand-operated buttons which are also part of the driver interface. In the OS-ASM mode the operator depresses the appropriate upshift or downshift paddle to indicate a desire for a gear shift; in the AS-ASM mode the demand for a gearshift is not under the operator""s control but rather under control of the system itself. For example, in the AS-ASM mode, the demand for a gearshift is produced by an engine control unit and is computed as a function of driver demanded torque and engine operating conditions.
In the prior art, the ASM gearshift event is performed analogously to that of a manual transmission except that the engine control unit aboard the vehicle controls: the intake throttle valve position and spark timing. A gearshift as a function of time, according to the prior art, is shown in FIG. 1 for a vehicle launch. A launch is when a resting vehicle is caused to drive away. At the left hand side of FIG. 1, the clutch is fully open. Shortly thereafter, a launch is requested and the clutch is caused to partially close. As a consequence of the clutch being partially closed, engine speed drops. Clutch pressure is controlled based on engine speed, as well as driver demanded torque. So, when engine speed starts to drop, the clutch is caused to retract to a fully open position. As a consequence of the clutch fully opening, engine speed flares. This continues in an unstable fashion causing the engine speed to deviate wider and wider. This vast fluctuation in clutch position and engine speed leads to an undesirable bucking of the vehicle. Typically, the driver intervenes, as was the case for that shown in FIG. 1, by adjusting the accelerator pedal position. If the driver depresses the accelerator pedal, the launch becomes a heavier launch and the condition can be exited. In the example shown in FIG. 1, engine speed fluctuations are mostly damped and the clutch is allowed to fully close by the end of the time shown in FIG. 1. In some situations, which require the most precise and slow vehicle maneuvers, such as positioning a car on a transporter for delivery, the operator of the vehicle desires a very slow vehicle speed. Thus, the driver intervenes by backing off the accelerator pedal and the clutch is returned to a fully open position. When the driver reapplies pressure to the accelerator pedal to attain a slightly higher vehicle speed, the bucking phenomenon recurs.
In a transition from the vehicle being at rest to moving, (i.e., a vehicle launch), it has been found that it is difficult to provide a reliably smooth launch, particularly when the operator is requesting a gradual launch, such as might be requested to undergo a parking lot maneuver.
In accordance with the present invention, a method and system are provided for controlling spark timing for spark plugs disposed in cylinders of an internal combustion engine. The method includes determining a base spark timing for such spark plugs. A time rate of change dNe/dt of engine speed is determined. The base timing is modified in accordance with the determined dNe/dt.
In accordance with another feature of the invention, a method and system are provided for use in controlling engine torque of an internal combustion engine, such torque being controlled by a spark timing of spark plugs disposed in engine cylinders. A base spark timing for such spark plugs is determined in an engine control unit electronically coupled to the spark plugs. A time rate of change dNe/dt of engine speed is determined. A spark timing offset from the base timing is determined for the spark plugs. The spark timing offset is a function of the determined dNe/dt. A new spark timing is determined for the spark plugs, such new spark timing being a function of the base spark timing and said offset spark timing. The engine control unit commands said new spark timing to the spark plugs.
According to another aspect of the invention, the spark timing offset is a function also of one or more of engine speed, engine coolant temperature, accelerator pedal position, a time rate of change of accelerator pedal position, a relative air charge, and engine speed divided by vehicle speed.
According to an aspect of the invention, the offset spark timing is set to zero when the time rate of change of engine timing is negative.
In one embodiment, a method and system are provided for controlling spark timing for spark plugs disposed in cylinders of an internal combustion engine of a vehicle. The method includes determining a launch requested by an operator of the vehicle. A determination is made of a base spark timing for such spark plugs. A time rate of change dNe/dt of engine speed is determined during the vehicle launch. The base timing is modified during the launch in accordance with the determined dNe/dt.
In one embodiment, the base spark timing is a spark advance which provides the best torque (MBT spark timing, discussed in more detail below). Alternatively, the base spark timing is retarded from MBT spark timing thereby providing a torque reserve.
The inventors of the present invention have found that the unstable vehicle launch can be prevented by a method for controlling torque in an internal combustion engine which is coupled to an automatic shifting manual transmission and installed in the vehicle. The torque is controlled by adjusting a spark timing of spark plugs disposed in engine cylinders. The method includes determining engine speed, and adjusting spark timing based on a derivative of engine speed.
More particularly, the inventors of the present invention have recognized the problem underlying the phenomena is that the point at which the clutch plates first touch, the kiss point, i.e., when there is first some torque delivery between the clutch plate on the engine side to the clutch plate on the transmission side, is not known. One solution would be to very slowly engage the clutch so that the kiss point could be detected. However, this is an unsatisfactory clutch engagement which leads to excessive clutch slippage resulting to excessive clutch wear and overheating. An advantage of the invention is that spark timing affects engine torque rapidly. Therefore, when an initial portion of an engine overspeed condition is detected, spark is adjusted immediately and it affects engine torque in the next cycle, thereby thwarting engine overspeed. Because of the fast actuation provided by adjusting spark timing, an unstable control during vehicle launch is prevented.